Information about the volume of urine in the bladder is important clinically for several types of patients suffering from bladder dysfunctions. For patients with neurogenic bladders, in whom the nervous system connections between the brain and the urinary bladder have been severed due to spinal cord injury or other disease conditions, sensations that normally alert individuals to the need for voiding a substantially filled bladder are either significantly reduced or absent altogether. Such patients are thus at risk of bladder overdistention, which is a condition that, if left untreated, can lead to permanent kidney damage and subsequent renal failure.
For patients with benign prostatic hyperplasia, which results in an enlarged or congested prostate, urine may be retained in the bladder after voiding. In a substantial percentage of such cases, the prostate growth constricts the urethra and thus restricts the flow of urine from the bladder. In response, the bladder grows thicker and stronger, to compensate for the increased resistance. Eventually, however, the bladder is no longer able to fully overcome the resistance and emptying becomes incomplete.
The retention of urine in the bladder puts the patient at risk of infection and bladder overdistention. Bacteria present in the urinary tract tend to multiply in the retained urine, and urinary infection can occur. The infection, in turn, can worsen the swelling already present in the prostate, and in the long run can lead to bladder stones or permanent kidney damage. The amount by which the prostate is enlarged correlates poorly with the degree of obstruction that the prostate presents to the flow of urine through the urethra. It is therefore important that patients with benign prostatic hyperplasia be monitored to determine the amount of retained urine after every voiding attempt.
Information about the volume of urine in the bladder is important also for individuals afflicted with urinary incontinence, especially overflow incontinence. Urinary incontinence is characterized by an involuntary loss of urine of sufficient quantity and/or frequency to become a social and health problem. There are two broad classes of incontinence, namely, transient and established. Transient incontinence, which is commonly caused by an acute illness or the administration of certain therapeutic drugs, is generally cured by treating the condition that caused the incontinence.
Established incontinence, on the other hand, is chronic and is generally caused by abnormalities in the function of the detrusor, the bladder outlet, or both. In many of these patients, the probability of a voiding accident increases sharply when the volume of urine in the bladder crosses an overflow threshold. While this threshold may vary from patient to patient, it is a widely held clinical belief that there exists a minimum overflow threshold that can be used as an effective predictor of such an accident in essentially every patient. Consequently, knowledge that the urine volume has crossed such a minimum threshold is useful in preventing leakage accidents.
Another class of patients for whom bladder volume information is important is patients undergoing lengthy surgery. In such patients, overdistention may occur as a result of anesthesia induced paralysis. Currently, these patients routinely have their bladders intubated with in-dwelling Foley catheters. The catheters are used to assess urine output and thereby monitor renal function and prevent bladder over distention. In-dwelling catheters, however, are major sources of infection. Furthermore, their use in urine output monitoring is labor intensive and cumbersome. Hence the use of a non-invasive and automatic process for this function is desirable.
Ultrasonic systems that use information contained in the backscattered echoes from the bladder region to determine its volume are known. One such system uses a specialized two-dimensional array transducer whose individual elements are activated simultaneously in two groups to produce first a composite transmit beam and then a composite receive beam. The signal associated with this composite receive beam is processed to extract three dimensional information about the bladder, which in turn is used to calculate its volume. To produce these two composite beams, the transducer has to be operated as a phased array; making it complex to manufacture, expensive and too bulky to be worn continuously.
Another known system uses an ultrasonic transducer that produces a single beam of ultrasonic energy. The signal received by the transducer contains information about the energy reflected from the back wall of the bladder, and the system uses an empirically developed calibration curve to indirectly relate this information to the degree of bladder distention. Consequently, the system produces only relative estimates of the bladder distention, rather than estimates of the bladder volume.
Yet another known system produces ultrasound measurements along two orthogonal planes that are each essentially normal to the direction of the patient's spine. Based on the measurements and an ellipsoidal model of the bladder, the system determines an estimate of bladder volume. The apparatus requires an operator to position and manipulate the associated transducer in a particular way in order to obtain the ultrasonic measurements, and thus, the system determines the bladder volume on an event-by-event basis. Accordingly, this system is not capable of automatically and/or essentially continuously producing the measurements necessary to produce estimates of bladder volume.